1. Field of the Invention
The invention relates to an image forming apparatus of an electrophotographic printing type provided with an intermediate transfer member to which a developer image is temporarily transferred.
2. Description of the Related Art
In recent years, a demand for improved productivity has been more and more increasing in an image forming apparatus for forming an image on a sheet such as plain paper by an electrophotographic printing method. As such an image forming apparatus of the eletrophotographic printing type, there is one on which is carried a both-side mode for effecting image formation on both of the front side and back side of a sheet. In order to improve productivity in this two-side mode, there has been proposed a so-called through-path duplex conveying method of effecting image formation while alternately feeding a sheet fed from a sheet feeding unit and a re-fed sheet having already had an image formed on one side thereof which is conveyed from a re-feeding path (duplex path) (see Japanese Patent Publication No. H07-37304).
In this through-path duplex conveying method, when the lack of sheets in the sheet feeding unit or the faulty conveyance of a sheet on a sheet feeding and conveying path occurs, the conveyance of the sheet is stopped. If at that time, a sheet present in the duplex path is in a state in which it can be discharged out of the machine, the formation of an image to be formed on the sheet in the duplex path is continued, and this sheet is intactly discharged out of the machine as a final printed matter. This dischargeable state is a case where the movement of the sheet in the duplex path is not hindered by a sheet stopped on any one path by the above-mentioned faulty conveyance. By doing such conveyance control, it is possible to minimize the possibility that a sheet being subjected to image formation may be left in a state stagnating in the machine.
In recent years, particularly as a color image forming method by the electrophotographic printing method, there is a method using an intermediate transfer member. This intermediate transfer member is a member to which a toner image developed on a photosensitive member is temporarily transferred, and the toner image transferred to this intermediate transfer member is further transferred to a sheet. This transfer of the toner image from the intermediate transfer member to the sheet is called an intermediate transfer process.
The use of such an intermediate transfer member makes it possible to achieve a decrease in color shift and the stabilization of transfer efficiency during transfer without being affected by the quality of material and thickness of the sheet. Also, particularly when an intermediate transfer belt is used as the intermediate transfer member, it becomes possible to make the arrangement and construction in the apparatus flexible. That is, the use of the intermediate transfer belt has a side that it is advantageous to the downsizing of the entire apparatus, and has been attracting attention as an apparatus form having various kinds of superiority.
However, in an image forming apparatus which does not use the intermediate transfer member, generally the time required to convey the sheet from the sheet feeding unit becomes longer than the time required for an image forming process and therefore, it is often the case that the image forming process is started later than the start of the conveyance of the sheet.
In contrast, in an image forming apparatus using the intermediate transfer member, it is necessary to execute the intermediate transfer process (secondary transfer) and therefore, the time required for the image forming process becomes long as compared with the image forming process which does not use the intermediate transfer member. When for example, the image forming process is started after the conveyance of the sheet has been started, the time required for a toner image to be transferred onto a sheet becomes long and as a whole, the throughput is reduced.
So, in order to prevent a reduction in the number of image-formed sheets per unit time, in the image forming apparatus using the intermediate transfer member, the image forming process on the photosensitive member is started prior to the feeding of the sheet. Then, a sheet conveying process is started so as to be in accordance with the timing for performing a secondary image transferring operation to be executed after the lapse of a predetermined time from the start of this image forming process.
Let it be assumed that in the image forming apparatus using such an intermediate transfer member, as described above, an abnormality such as the lack of sheets in the sheet feeding unit or the faulty conveyance of the sheet has occurred when the through-path duplex conveyance of the sheet is being performed. It is conceivable that at the point of time whereat this abnormality has occurred, both of an image for the front side to be transferred to a sheet of which the conveyance has been interrupted and an image for the back side to be transferred to a sheet fed from a duplex path have already been transferred onto the intermediate transfer member.
Particularly when the image being transferred onto the intermediate transfer member prior to the feeding of the sheet is an image for the front side, it is necessary to once clear all toner images formed on the intermediate transfer member including the succeeding image for the back side. Consequently, even when for example, the lack of sheets has occurred or when a faultily conveyed sheet is stopped at a position whereat it does not hinder re-feeding from the duplex path, if the secondary transfer portion is spaced apart, it is impossible to effect the discharge of the sheet on the duplex path. As the result, when the lack of sheets has occurred or when the faulty conveyance on the sheet feeding path has occurred, the operation is interrupted in a state in which the sheet on the duplex path originally dischargeable as a final printed matter is stagnated in the machine.
Accordingly, the sheet on which an image is still being formed becomes stagnated in the machine and therefore, the processing for removing the stagnated sheet in the machine after the interruption of the operation becomes cumbersome, and a user's operating property is reduced. Also, the sheet originally continuedly subjected to image formation on the back side thereof and dischargeable out of the machine as a final printed matter is stopped in its unfinished state in the machine, thus resulting in a reduction in productivity and an increase in running cost due to the loss of the toner and sheets accompanying the interruption of image formation.